Ch. 2 Ventilation (How gas gets to the alveoli)
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Various bronchi that make up the conducting airways can be represented by a single
tube labeled – anatomic dead space
Typical Volumes and flow w/in the lungs
o Tidal Volume = 500 ml
 With each inspiration about 500 ml of air enter the lung
o Total ventilation = 7500ml/min
o Anatomic dead space = 150 ml
o Frequency = 15/min
o Alveolar ventilation = 5250 ml/min
o Pulmonary blood flow = 5000 ml/min
 Alveolar vent. / pulmonary blood flow ~= 1
o Alveolar gas = 3000 ml
o Pulmonary capillary blood = 70ml
Lung volumes based on a spirometer
o Note: total lung capacity, fx’al residual capacity, and residual volume cannot
be measure with a spirometer
o Total Lung capacity ~ 7L
o Vital capacity ~5L
o Tidal volume ~ ½ L
o Functional residual capacity ~3L
Lung Volumes
C1 x V1 = C2 x (V1 + V2)
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Vital Capacity: the exhaled volume
Residual Volume: the amt. of gas that remains in the lung after a maximal expiration
Functional residual capacity: the volume of gas in the lung after a normal expiration
o Neither the functional residual capacity nor the residual volume can be measured with
a simple spirometer
o Helium dilution method
 Measures only communicating gas, or ventilated lung volume
 How to calculate the previous listed: patient inhales a small amt. of helium, and
helium concentrations in the spirometer and lung become the same
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b/c no helium is lost; the amt. of helium present before equilibration
(concentration times volume) is:
o C1 x V1
 After equilibration:
o V2 = V1 (C1 – C2) / C2 or:
 C1 x V1 = C2 x (V1 + V2)
Body plethysmograph
 Another way of measuring the fx’al residual capacity (FRC)
 Measures the total volume of gas in the lung, including any that is trapped
behind closed airways
 A large airtight box (like telephone booth)
 At the end of a normal expiration, a shutter closes over a mouthpiece and
subject is asked to make respiratory efforts. As the subjet tries to inhale, he
expands the gas in his lungs, and lung volume increases, and the box pressure
rises b/c its gas volume decreases
 Boyle’s Law: pressure x volume is constant (at constant temp.)
 P1 = pressure in box before inspiratory effort
 P2 = pressure in box after inspiratory effort
 V1 = preinspiratory box volume
 Delta V = change in volume of the box/lung
 P1 V1 = P2 ( V1 – delta V)
 W/ Boyle’s Law applied to gas in the lung:
 P3 V2 = P 4 ( V2 + delta V)
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Where P3 and P4 are the mouth pressures before and after the
inspiratory effort, and V2 is the FRC
 Thus FRC can be obtained
o In young normal subjects – ventilated lung volume ~= communicating gas
o In lung disease – ventilated volume may be considerably less than the total volume b/c
of gas trapped behind obstructed airways.
Lung Volume summary:
o Tidal volume and vital capacity can be measured with a simple spirometer
o Total lung capacity, fx’al residual capacity and residual volume need an additional
measurement by helium dilution or the body plethysmograph
o Helium is used b/c of its very low solubility in blood
o Body plethysmograph depends on boyle’s Law PV =K at constant temp.
Ventilation
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Suppose:
o Volume exhaled w/ each breath is 500 ml
o RR = 15 breaths/min
 Total volume leaving the lung each minute  500 x 15 = 7500 ml/min (total
ventilation)
o Total Ventilation: the volume of air entering the lung is very slightly greater b/c more
oxygen is taken in than carbon dioxide is given out
o Anatomic Dead space: about 150 ml from each 500 ml of air inhaled enters the
anatomic dead space (30%)
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Alveolar Ventilation: the volume of fresh air entering the respiratory zone each minute is (500 –
150) x 15 = 5250 ml/min
o Represents the amount of fresh inspired air available for gas exchange (specifically, the
alveolar ventilation is also measured on expiration, but the volume is almost the same)
o How to determine alveolar ventilation:
o 1) measure the volume of the anatomic dead space and calculate the dead space
ventilation (volume x respiratory frequency); this is then subtracted from the total
ventilation
 V=volume, T=tidal, D=dead, A=alveolar
 VT = VD + VA (VT x n = VD x n + VA x n) where n = respiratory frequency
 Or,
 VE = VD + VA where:
 VE= expired total ventilation
 VD=dead space
 VA= alveolar ventilation (vol. of alveolar gas in the tidal volume NOT the
total volume of alveolar gas in the lung)
 Or VA = VE - VD
 Note: the alveolar ventilation can be increased by raising either tidal volume or
respiratory frequency
 Increasing the tidal volume is often more effective b/c this reduces the
proportion of each breath occupied by the anatomic dead space
o 2)measure the concentration of CO2 in expired gas
 Because all expired CO2 comes from the alveolar gas
 VCO2 = VA x ( % CO2 / 100)
 Or VA = (VCO2 x 100 / % CO2)
 Fractional concentration: %CO2/100  denoted as FCO2
 Thus, alveolar ventilation can be obtained by dividing the CO2 output by the
alveolar fractional concentration of this gas
 Note: partial pressure of CO2 (denoted PCO2) is proportional to the fractional
concentration of the gas in the alveoli
 i.e.: PCO2 = FCO2 x K
 or: VA = (VCO2 / PCO2) x K
 Tidal Volume (VT) is a mixture of gas from the anatomic dead space (VD) and a
contribution from the alveolar gas (VA).
 b/c PCO2 of alveolar gas and arterial blood are identical: the arterial PCO2 can be
used to determine alveolar ventilation
 If the alveolar ventilation is halved (and CO2 production remains unchanged),
the alveolar and arterial PCO2 will double
Anatomic Dead Space
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Normal value = 150ml
Increases w/ large inspirations
Depends on size and posture of subject
Measured by Fowler’s method
o Measures the volume of the conducting airways down to the level where the rapid
dilution of inspired gas occurs with gas already in the lung (and b/c it reflects
morphology of lung  anatomic dead space
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Following single inspiration of 100% O2, the n2 concentration rises as the dead space
gas is increasingly washed out by alveolar gas
o Uniform gas concentration is seen representing pure alveolar gas
 Alveolar plateau (in normal subjects not quite flat)
 w/ lung disease may rise steeply
found by plotting N2 concentration against expired volume
o and then connecting points of N2 concentration almost at plateau (@ pure alveolar gasA) and at expired volume-B,
o the vertical dashed line = dead space
Physiologic Dead Space
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Bohr’s method / Bohr equation
o All CO2 comes from the alveolar space, NOT from the Dead Space Vol.
o Measures the volume of the lung that does not eliminate CO2
o Fx’al measurement  vol. is called physiologic dead space
o Show’s that all the expired CO2 comes from the alveolar gas and none from the dead
space
o Normal ratio of dead space to tidal volume is in the range of 0.2 to 0.35 during resting
breathing
Ventillation Summary
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Total ventilation is tidal volume x respiratory frequency
Alveolar ventilation is the amt. of fresh gas getting to the alveoli, or (VT - VD) x n
Anatomic dead space is the volume of the conducting airways, about 150 ml
Physiologic dead space is the vol. of gas that does not eliminate CO2
The 2 dead spaces are almost the same in normal subjects, but the physiologic dead space is
increased in many lung diseases
Difference b/t Bohr and Fowler – in normal subjects anatomic dead space ~= physiologic dead
space
o In pt’s w/ lung disease:
 Physiologic dead space >> anatomic dead space
 b/c of inequality of blood flow and ventilation w/in the lung
Regional Differences of Ventilation
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Ventillation  lower lungs >> upper lungs
When subject is in supine positon, this difference disappears w/ the result thatpical and basal
ventilations become the same
o However, when supine  the ventilation of the posterior lung >> anterior lung
o If subject lateral  dependent lung is best ventilated